This invention relates to water purification and more specifically, the invention relates to purifying water containing contaminants such as natural organic matter, bacteria, color, turbidity and insoluble impurities to provide potable water.
Drinking water sources high in natural organic matter, bacteria, color, turbidity and insoluble impurities are often treated with coagulants to form a settable floc which is removable by sedimentation. The insoluble particles are often removed by sand or multi-media filtration to remove fines. However, these systems do not provide adequate removal of cysts such as Giardia and oocysts such as crystosporidium. Further, these systems are often inadequate in achieving sufficient removal of organics to meet increasingly stringent requirements of disinfection by-products (DBP), resulting from the reaction of organic matter with chlorine disinfectant. Further, the resulting reaction products can be toxic.
Nanofiltration has been used to reduce organic materials to concentrations which results in acceptable DBP levels. While this can remove most of the offending microorganisms, the process is complex and expensive.
A number of microfiltration and ultrafiltration systems have been used on a commercial scale to remove low levels of turbidity, cysts, oocysts and bacteria in potable water. However, none of these systems are capable of processing waters with high levels of turbidity, color, organic matter or virus. If microfiltration is used, water must be post-treated with nanofiltration to remove color and organics. These process configurations, with either pre- or post-treatment, in conjunction with microfiltration and ultrafiltration, result in expensive systems which are difficult to operate. When coagulants are used, the concentration of solids in the concentrate must be kept low (up to 300 ppm), because higher concentrations either plug the membrane or result in a significant loss of operating time because of frequent backwash requirements with a resultant loss in operating capacity.
In an article by Fu and Dempsey entitled "Effects of Charge and Coagulant Dose on NOM Removal and Membrane Fouling Mechanisms" (pp. 1043-1058), presented at the American Water Works Association, Membrane Technology Conference Proceedings, February 23-26, 1997, New Orleans, La., the effect of addition of calcium to water containing natural organic matter on membrane flux was studied. It was determined that flux through membrane dropped as the level of calcium increased. Thus, when calcium dosage was increased from 0 to 50 mg/l, a 55% reduction in membrane flux was observed. This was attributed to the deposition of calcium and hydrophobic organic compounds on the membrane surface thereby blocking off the pores.
Different processes have been proposed for treating waste waters. For example, U.S. Pat. No. 5,480,537 discloses an apparatus for waste water treatment, wherein fluorine contained in waste water is converted into a fluoride through a chemical reaction. Sludge containing the fluoride is thickened and dehydrated into a sludge cake. The apparatus has first and second tanks each packed with calcium carbonate mineral. In the first tank, waste water containing fluorine is agitated by air discharged from an air diffuser and caused to react with calcium carbonate mineral.
U.S. Pat. No. 4,390,431 discloses a process for treating an aqueous phase obtained by acidic or basic washing of a hydrocarbon phase comprising a soluble aluminum compound in solution. The process is characterized in that it is neutralized at a pH of from 6 to 8.5, treated with an additive selected from the alkaline-earth metal oxides, hydroxides and carbonates, at a temperature ranging from 90.degree. C. to the boiling point, the molar ratio of the additive to aluminum being from 1/10 to 10/1, and separated thereafter from the insoluble phase of precipitate.
Japanese Patent Application 9500963 discloses that before the ultraprocessing of the electrodialysis treatment and the reverse osmosis membrane treatment is administered to the preceding untreated water, the untreated water, flocculant and the pH adjustment agent are drawn to separation equipment. The coagulation of the water to be treated is done in the separation tank. The pretreatment method in this ultraprocessing is characterized by activated carbon adsorption and water softening treatment which are administered to the filtered water. During the immersion, aspiration filters the water with the membrane separation equipment.
Japanese Patent JP 7060248 discloses that the turbidity particles in the raw water are coagulated with aluminium salt solution. This is done in the reverse osmosis membrane facility. The resulting water is filtered by prefiltration prior to the reverse osmosis membrane facility using a precision filtration membrane. According to this reference, it is possible to filter out unwanted chemicals including iron, magnesium, organic matter or the like with the precision filtration membrane and plugging of the filtration membrane never occurs. High filtration accuracy can be obtained, extending the lifetime of the reverse osmosis membrane in the reverse osmosis facility.
In spite of these processes, there is a great need for an economical process for purifying water which will effectively remove constituents such as natural organic matter, bacteria, color, turbidity, cysts and oocysts, viruses, arsenic compounds and insoluble impurities to a very low level to provide potable water. The subject invention provides such a process.